This invention relates in general to fuses and more particularly to high speed fuses.
High speed fuses have been used for a number of years for the protection or isolation of semiconductor devices such as diodes and thyristors. There is very little safety factor in these semiconductor devices and they can fail quickly when subjected to overcurrents. Therefore, a fuse designed to protect semiconductor devices must open quickly. High speed fuses have very little thermal capacity, and in general open in the order of 0.001 to 0.004 seconds when interrupting short circuits.
Problems exist with high speed fuses currently on the market because these fuses have been developed over time to meet specific applications, resulting in a large number of different fuses made in different sizes and shapes to satisfy the voltage and amperage ranges expected to be encountered. Several hundred different parts and subassemblies for these fuses may be required. Thus, it would be desirable to be able to manufacture fuses having standardized parts to reduce the total number of parts that need to be stocked in order to manufacture a complete line of high speed fuses.
Many applications for high speed fuses require the use of a plurality of fuses usually mounted side by side in close proximity of one another. Many users wish to mount multiple fuses as close together as possible. Prior art high speed fuses have metal end bells which are mechanically and thus electrically connected to the mounting terminals held to the insulating tube with metal pins which are exposed flush with the tube surface and are not sealed. Consequently, when in use in an electrical circuit, the pins are at the same electrical potential as the terminals and end bells. Typically, three phase electrical applications use a fuse in each phase mounted adjacent to each other and as close as possible to conserve space within the equipment. Industrial standards govern minimum spacing between electrically hot parts and dictate the minimum distance between live or "electrically hot" parts through air as a function of stand-off voltage. Since the pins are electrically hot and exposed to the tube surface, this minimum distance is measured between adjacent tube surfaces, as opposed to terminal distances.
Yet another difficulty is encountered in manufacturing high speed fuses in that the end bell must be joined to the terminal for mechanical strength of the fuse package and, in most designs, for the electrical connection between the current carrying fusible elements within the fuse and the mounting terminal. Prior art high speed fuses accomplished this by brazing, welding or soldering the terminal to the end bell or machining the end bell and terminal from a solid piece of metal or by pressing the metal pins through the tube and end bell and into the mounting terminal. All these techniques are labor intensive.
A further problem is encountered with end bells in that these circular pieces of metal are most often forged or machined from rod stock and coined, drilled, and sized. This again requires extra time and additional labor and is thus more expensive.
Yet another manufacturing problem is encountered in making high speed fuses. These fuses, in general, are filled with sand or other arc quenching materials. This material is added through a hole in the end bell after the end bell is assembled to the fuse tube. Various methods of plugging the hole have been used, but all suffer from various limitations. For example, costly knurled plugs are used which require excessive pressure to insert the plug into the hole in the end bell. A more economical means to close the sand hole is required.